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Please re-add the feed again, but when you'll see the yellow "Importing from URL" message - close the calendar. Google will then automatically sync everything within a couple of hours.

If you did add the feed already, but don't see new tasks that you're adding to Todoist show up in the calendar then unfortunately, this is a limitation on Google's side as they're syncing the iCal feed only once a day so you will see the changes with a delay.